This invention generally relates to Q-switched lasers in which the rear mirror element is a rotating type mirror element. More specifically, this invention relates to double pulse suppression in such lasers by the provision of a pulse suppression optical discontinuity within the laser cavity. More specifically, this invention relates to such a laser in which the pulse suppression discontinuity is provided by a bevel portion on the laser rod element within the laser cavity.
Q-switched lasers are well known in the art. Such lasers are designed to delay the generation of the amplified lasing pulse until such time as the lasing medium has been pumped or activated to its maximum possible energy level. To achieve this purpose, Q-switched lasers have a rear mirror that is blocked from view of the laser rod for a short period of time during the pumping of the laser rod by a flash tube. It is therefore not possible for the pulses which exit along the axis of the rod to be reflected back into the rod and cause amplification which eventually leads to the laser pulse. At such time as the mirror does come into alignment with the rod, a laser pulse is generated within a very short time frame which gives maximum power release. However, this theoretical consideration is somewhat distorted by the practicalities of realistic operating lasers. The use of rotating rear mirror elements is common in many Q-switched type lasers. This is because the electrically switchable cells are quite fragile, expensive, and difficult to operate. The rotating type rear mirror system does, however, lead to a problem known as double pulsing. In this situation, it is possible for very shallow angle photon beams to be reflected from the surface of the rotating mirror before it has come into complete alignment with the axis of the laser rod. What occurs is that the shallow angle photon beams are reflected back and forth from the rear mirror along the rod to the front mirror and back again at these shallow angles along the edges of the laser rod. It is possible to achieve sufficient amplification in this initial mode to generate a laser pulse. Then, when the mirror has come into its desired alignment position, the laser pulse which is desired is generated. It should thus be clear that two pulses actually occur, double pulsing, which is undesirable since part of the energy which has been stored is dissipated in the initial pulse, and the two pulses make it difficult to use the desired single pulse for measurement purposes. The prior art has attempted to overcome this problem by positioning a metal element such as a knife or razor edge into the path of travel of such shallow angle reflected photon beams adjacent to the rear mirror. While this has had the effect of significantly reducing the double pulsing phenomenon, when the main pulse is generated enough of the laser beam is scattered to cause damage to the razor edge. Small fragments of metal are vaporized and thrown off from this edge as a result of contact by the laser beam. This vaporized metal is then deposited on the optical surfaces within the laser cavity leading to degrading the performance of the laser. I have found that if an optical discontinuity is provided on one of the optical surfaces defining the laser cavity, it is possible to prevent or block propagation of the off axis shallow angle photon beams. This occurs by virtue of the fact that the beams may be either completely internally reflected, reflected away from the rod, or diffusely scattered to such an extent that they are unable to re-enter or leave the laser rod itself and thus lead to quenching of the amplification action within the rod. As one specific example of such an optical discontinuity, a bevel portion may be cut across one face of the laser rod at any angle which is preferably at least as great as the critical angle of the material from which the laser rod is formed. This bevel must be positioned so that it faces in the direction into which the rotating rear mirror element opens.